What can be learned from wild populations of pests to better manage resistances in the fields?

Session 2: Agricultural Ecology Pests and PesticidesFollowing the ban of pesticide treatments, resistant varieties have emerged as one of the best environmental-friendly methods to control populations of parasitic nematodes. However, as host resistances exert strong selective pressures on parasites populations, they are likely to be overcome more or less rapidly, according to the effective sizes of parasite populations and to the level of gene flow among them. In agricultural landscapes, gene flow is mainly the result of human activities. Hence, to better manage these pests in agricultural fields, these two parameters should be evaluated in non-disturbed wild populations. The cyst nematode Heterodera schachtii is growing both on cultivated beet, in the fields, and on its wild relative, the sea beet. To characterize effective sizes and levels of gene flow in the wild, we assessed the temporal variation of allele frequencies at microsatellite loci from two successive samples taken one year apart on individual plants from different beaches. According to the environmental conditions of the studied areas, the number of generations produced by Heterodera schachtii between the two samplings was estimated as ranging from 4 to 10. Patterns of genetic diversity observed were incompatible with the occurrence of efficient gene flow, even between neighboring host plants, which resulted in low effective population sizes with respect to the actual number of individuals found in each rhizosphere of host plants. These low effective population sizes and the inability of the parasite to actively disperse over large distances suggested very limited possibilities to overcome resistances, provided that the passive transport of cysts by agricultural machinery, and by spreading sludge processing from sugar factories, are themselves limited